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Using coal rank data and the demonstrated genetic link between the present hydrodynamic regime and geothermal gradient pattern, it is possible to reconstruct the geothermal history of Alberta over the past 60 m.y. Palinspastic adjustment for tectonic compression caused by the major Laramide thrusting shows that the predeformation isoreflectance lines increased logarithmically with depth. In the late Paleocene, the geothermal gradient was about 23°C/km (1.25°F/100 ft) in the eastern Alberta Plains, compared to about 30°C/km (1.65°F/100 ft) in the western Alberta Plains, a regional trend opposite to the post-Laramide trends. Reconstruction of the early Eocene surface indicates western uplands with geothermal gradients as low as 21°C/km (1.15°F/ 00 ft) and eastern lowlands with geothermal gradients of 27°C/km (1.5°F/100 ft). Compared to the present situation, this represents an enhanced topographic surface and a subdued geothermal gradient pattern.
The genetic relations of topography (water-table elevation), hydrodynamic regime, and geothermal gradient pattern in both the early Eocene and the present conform to a model developed for any compacted sedimentary basin with subaerial relief. In this model, on a regional scale, high topographic areas have high water-table elevations with correspondingly high potentiometric surfaces; these areas control the regional recharge of cold meteoric water and hence have low geothermal gradients. Areas of medium elevation exhibit regional lateral flow and intermediate geothermal gradients. Regional topographic lows correspond with low potentiometric surfaces, and the regional discharge of warm formation waters from deep in the basin results in high geothermal gradients. Local topographic featur s cause perturbations in this picture. Study of both the regional pattern and fine detail of the early Eocene and present topographic surfaces and geothermal gradient patterns shows the validity of the model. The result of the second Laramide orogenic pulse was, therefore, to reverse the geothermal gradient pattern resulting mainly from compaction flow, and to impose a topographically controlled hydrodynamic regime with flow in the same direction but with a 9°C/km (0.5°F/100 ft) decreased geothermal gradient in western Alberta and a 4°C/km (0.2°F/100 ft) increased geothermal gradient in eastern Alberta.
As erosion proceeded during the Tertiary, the entire Alberta basin warmed up until the present temperature regime was attained. Study of some major hydrocarbon occurrences shows a genetic link between the hydrocarbon occurrence and the present fluid regime which itself is controlled by the present topography. For at least the significant Upper Cretaceous shallow gas reserves in southeastern Alberta, however, their position may already have been determined as early as the beginning of the Eocene by the topographic surface at that time. Therefore, the extent to which the present topographic surface is responsible for hydrocarbon accumulation through gravity-induced cross-formational flow remains unknown because this process was operative as long ago as the early Eocene. What does not ap ear to be in doubt is the fact that gravity-induced cross-formational flow does control the hydrodynamic regime, which in turn influences the geothermal gradient pattern and the accumulation of hydrocarbons.
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